Mould for casting electrode grids for electric storage batteries

ABSTRACT

A mould for casting lead or lead alloy electrode grids for electric storage batteries comprises backing plates of metal faced with inserts of silicon nitride exposed to the melt.

United States Patent 1 Bushrod 51 Jan. 9, 1973 [54] MOULD FOR CASTING ELECTRODE GRIDS FOR ELECTRIC STORAGE BATTERIES [75] Inventor: Charles James Buslirod, Bramhall,

England [73] Assignee: Electric Power Storage Limited, Clifton Junction, Lancashire, En gland 221 Filed: Nov. 19, 1970 21 Appl. No.: 90,889

[30] Foreign Application Priority Data Primary Examiner-R. Spencer Annear Attorney-Watson, Cole, Grindle & Watson [57] ABSTRACT NOV. 21, Great Britain A mould f sting lea r lead al oy lectrode g ids for electric storage batteries comprises backing plates [52] US. Cl ..249/134, 164/138 of metal faced with inserts of silicon nitride exposed to [51] Int. Cl ..B22c 1/12 the melt i o [58] Field of Search ..164/138; 249/114, 134;

l17/5.1 8 Claims, 1 Drawing Figure I IHHHH H MOULD FOR CASTING ELECTRODE GRIDS FOR ELECTRIC STORAGE BATTERIES This invention relates to casting lead or lead alloy electrode grids for electric storage batteries.

According to the present invention in a method of casting lead or lead alloy electrode grids for electric storage batteries at least the faces of the mould exposed to the melt comprise silicon nitride. Thus the mould may comprise inserts of silicon nitride backed by backing plates of metal such as cast iron.

In one form of the invention a method of making such a mould includes pressing silicon powder between punches at least one of which has in it a form corresponding to the desired casting, and thereafter heating it in an atmosphere of nitrogen.

An object of the invention is to enable lead or lead alloy battery plate grids to be cast without using mould dressing. A customary material for moulds for making such castings is meehanite cast iron, and the usual procedure is to spray the face of the mould with cork dust, generally about twice in each shift, in order to ensure filling of the mould. If such dressing is overlooked the mould is liable to fail to fill, and a large number of rejects may be obtained before the oversight is detected.

Although silicon nitride has a low co-efficient of expansion and, for a ceramic, a relatively high thermal conductivity, resulting in high resistance to thermal shock, its thermal conductivity compared with metals is relatively low. This is of substantial advantage in casting thin lead castings such as battery plates which consist virtually of a network of wires and involve liquid metal traversing long narrow passages in order to reach all parts of the casting. Due, it is believed, to the low thermal conductivity of silicon nitride, it is found that the mould dressing can be omitted without producing a faulty casting that would result from this with a cast iron mould. V

In addition the cork dressing assists the casting to part from the mould. It is found that the casting parts readily and clearly from the mould face of silicon nitride so that in this respect also the use of this material for the facing of the mould eliminates the need for mould dressing.

Silicon nitride is sufficiently hard to stand up to the wear involved.

In some cases it may be found that the reduction of thermal conductivity is more than is required and the cooling of the casting may be unduly delayed. In this event the cooling of the mould may be increased by providing a layer of metal foil, (e.g. copper foil), of high conductivity, between the silicon nitride inserts and the backing plates.

The invention may be put into practice in various ways but one specific embodiment will be described by way of example with reference to the accompanying drawing which is a sectional elevation of a mould for casting lead or lead alloy electric storage battery plate grids.

The mould includes a pair of silicon nitride facing plates 10 and 11 sandwiched between a pair of meehanite cast iron backing plates 16 and 17. Each facing plate is bevelled along two or more edges, and correspondingly bevelled clamping plates 24 and 25 are screwed, by screws 20 and 21 to the associated backing plate 16 or 17 to hold the facing plate in position. Each facing plate is provided with venting holes 12 and 13 and asbestos paper seatings 18 and 19 are interposed between the opposed faces of the backing plates 16 and 17 and facing plates 10 and 11. The usual alignment dowels 22 extend through the backing plates 16 and 17, in which they fit, and through the clamping plates 24 and 25 in which they have a clearance.

If undue delay is experienced in the warming up of the mould when casting is started, the mould may be pre-heated.

It is believed that, perhaps by eliminating the need for dressing, the use of silicon nitride moulds gives improved surface finish to the castings, resulting in reduced corrosion rate of grids and improved overcharge life.

What we claim as our invention and desire to secure by Letters Patent is:

l. A mould for casting lead or lead alloy electrode grids for electric storage batteries in which the faces of the mould exposed to the melt comprise inserts of silicon nitride which are backed by backing plates 0 metal.

2. A mould as claimed in claim 1 in which the backing plates are of cast iron.

3. A mould as claimed in claim 1 which includes a layer of metal foil, of high conductivity, between the silicon nitride inserts and the backing plates.

4. A mould as claimed in claim 3 in which the metal foil is copper foil.

5. A mould as claimed in claim 1 which includes asbestos paper between the silicon nitride inserts and the backing plates.

6. A mould as claimed in claim 2 which includes asbestos paper between the silicon nitride inserts and the backing plates.

7. A mould as claimed in claim 3 which includes asbestos paper between the silicon nitride inserts and the backing plates.

8. A mould as claimed in claim 4 which includes asbestos paper between the silicon nitride inserts and the backing plates. 

2. A mould as claimed in claim 1 in which the backing plates are of cast iron.
 3. A mould as claimed in claim 1 which includes a layer of metal foil, of high conductivity, between the silicon nitride inserts and the backing plates.
 4. A mould as claimed in claim 3 in which the metal foil is copper foil.
 5. A mould as claimed in claim 1 which includes asbestos paper between the silicon nitride inserts and the backing plates.
 6. A mould as claimed in claim 2 which includes asbestos paper between the silicon nitride inserts and the backing plates.
 7. A mould as claimed in claim 3 which includes asbestos paper between the silicon nitride inserts and the backing plates.
 8. A mould as claimed in claim 4 which includes asbestos paper between the silicon nitride inserts and the backing plates. 